The present invention relates to the use of amphiphilic copolymers to stabilize dispersions of insoluble organic compounds comprising at least one group that absorbs UV radiation, which are in the form of particles with a mean size of between 10 nm and 5 xcexcm, to the dispersions obtained, stabilized with these copolymers, and to cosmetic compositions containing them, and also to a process for preparing such dispersions.
Many cosmetic compositions for photoprotecting the skin have been proposed to date.
These antisun compositions are often in the form of an emulsion of oil-in-water or water-in-oil type, of gels or of anhydrous products containing one or more organic and/or mineral, insoluble and/or soluble, lipophilic and/or hydrophilic screening agents capable of selectively absorbing harmful UV radiation. The efficacy of these screening agents is characterized by their molar extinction coefficient xcex5. The nature and concentration of the screening agents in the antisun compositions are chosen as a function of the desired protection factor and the desired protection profile. Depending on their lipophilic or hydrophilic nature, the screening agents may be distributed either in the fatty phase or in the aqueous phase of the antisun composition.
A large number of organic screening agents commonly used bear substituents that make them soluble in various cosmetic media: chains of 2-ethylhexyl type, for example, make screening agents soluble in oils, and ionized groups such as sulphonic acid give them a certain level of solubility in polar solvents and in particular in water.
However, it has been observed that the presence of substituents on the chromophore results in a reduction in the absorbing power, and there is thus great interest in being able to use screening agents containing an unsubstituted chromophore in order to fully exploit their absorption properties.
However, the insolubility of these screening agents in most cosmetic solvents makes them difficult to handle. It has moreover been found that, in order to be fully effective, they must be in the form of very fine particles fully dispersed in the cosmetic medium.
One of the solutions for obtaining a satisfactory dispersion of particles of screening agents in cosmetic compositions consists in first producing a concentrated predispersion that will then be introduced into the cosmetic medium.
Such predispersions of micronized insoluble organic screening agents may be prepared by grinding up a dispersion of coarse particles in the presence of one or more suitable dispersants, to the desired particle size. These dispersants are generally compounds with interfacial activity, which position themselves at the solid/liquid interface, thus preventing the aggregation and sedimentation of the particles.
International patent application WO 97/03643 (corresponding to U.S. Pat. No. 5,980,872) and patent application EP 0 893 119 (corresponding to U.S. Pat. No. 6,235,271) describe a process for grinding (micronizing) insoluble organic screening agents in the presence of a dispersant chosen from alkylpolyglucosides of formula CnH2n+1O(C6H10O5)xH in is which n is an integer between 8 and 16 and x is between 1.4 and 1.6.
International patent application WO 95/22959 (corresponding to U.S. Pat. No. 5,869,030) discloses a process for micronizing insoluble organic screening agents in the presence of a grinding adjuvant chosen from alkylated polyvinylpyrrolidones, copolymers of vinylpyrrolidone and of vinyl acetate, acylglutamates, copolymers of acrylic acid and of tert-octylpropenamide, ditolylethersulphonic acid/formaldehyde condensation products, Carbomer(copyright) products, a commercial mixture of fatty acid esters comprising an ethoxylated tristyrylphenol and phospholipids.
The use of the above dispersants and grinding adjuvants to stabilize dispersions of micronized organic UV screening agents nevertheless poses a certain number of problems. Specifically, although these dispersants can produce stable concentrated predispersions, the dilution of the predispersions in the cosmetic medium during the preparation of the antisun compositions is generally accompanied by an at least partial desorption of the dispersants and a destabilization of the final antisun cosmetic composition.
Attempts have been made to overcome this instability observed at the time of dilution of the predispersions by increasing the concentration of dispersants. However, it has been found that such an increase in the concentration had the consequence of harming the persistence of the insoluble organic screening agents on the skin or the hair, i.e. of promoting their removal on contact with water.
Consequently, the aim of the present invention is to find dispersants for stabilizingxe2x80x94at relatively low concentrations that do not harm the remanence of the screening agentsxe2x80x94both concentrated dispersions of micronized insoluble organic screening agents and the cosmetic compositions obtained by diluting these concentrated dispersions in a cosmetic medium.
The inventors have found that one particular group of polymers having the common characteristic of comprising at least one block of hydrophilic monomers and at least one block of hydrophobic monomers, known as xe2x80x9camphiphilic copolymersxe2x80x9d, makes it possible to satisfactorily stabilize concentrated dispersions and dilute dispersions of fine particles of insoluble organic compounds that absorb UV radiation.
Consequently, one subject of the present invention is the use of at least one amphiphilic copolymer comprising at least one hydrophilic block and at least one hydrophobic block, to stabilize dispersions of insoluble organic compounds comprising at least one group that absorbs UV radiation, which is in the form of particles with a mean size of between 10 nm and 5 xcexcm.
A subject of the invention is also a process for preparing such dispersions, which consists in subjecting particles of the said organic compound, suspended in a liquid medium, to a step of particle size reduction, until a mean particle size of between 10 nm and 5 xcexcm is obtained, and in adding, before, during or after the step of particle size reduction, at least one amphiphilic copolymer comprising at least one hydrophilic block and at least one hydrophobic block.
A subject of the invention is also dispersions comprising, in a liquid medium, at least one insoluble organic compound comprising at least one group that absorbs UV radiation, in the form of particles with a mean size of between 10 nm and 5 xcexcm, and at least one dispersant chosen from a particular group of amphiphilic copolymers comprising at least one hydrophilic block and at least one hydrophobic block, and also cosmetic compositions for topical application, in particular compositions for photoprotecting the skin and/or the hair, prepared by diluting its dispersions in a cosmetically acceptable medium.
In contrast with the dispersions of micronized organic UV screening agents, stabilized with the dispersants of the prior art (see WO 97/03643 and WO 95/22959), the dispersions of the present invention stabilized with the amphiphilic polymers described in greater detail hereinbelow remain entirely stablexe2x80x94i.e. no aggregation, flocculation or sedimentation of the particles is observedxe2x80x94when cosmetic compositions are prepared from these dispersions by dilution in a cosmetically acceptable medium. The stabilizing effect, observed at the time of dilution, is obtained for small amounts of dispersing polymers, i.e. of the order of a few milligrams per square metre of surface area of particles only, which not only reduces the costs of the starting materials for the compositions, but above all has the advantage of not impairing the remanence of the insoluble UV screening agents on the skin and/or the hair.
The photoprotective cosmetic compositions containing particles of insoluble organic UV screening agents, stabilized with the amphiphilic polymers described below, are moreover distinguished by an excellent photoprotective power and satisfactory cosmetic properties.
In the present invention, the expression xe2x80x9camphiphilic copolymer comprising at least one hydrophilic block and at least one hydrophobic blockxe2x80x9d means polymers comprising at least two types of monomer xe2x80x9cblockxe2x80x9d, each of the blocks consisting of a succession of identical monomers. The amphiphilic polymers of the present invention may have a linear structure in which the various blocks are linked together, alternately (hydrophilic/hydrophobic), one after the other. This type of polymer including diblock, triblock or polyblock linear structures is generally known in the field of macromolecular chemistry as xe2x80x9cblock copolymersxe2x80x9d. However, the investors also intend to include in the definition of the amphiphilic copolymers used in the present invention polymers having a branched structure, for example a structure in which blocks of a first type (hydrophobic or hydrophilic) are attached to a main chain forming a second type of block (hydrophilic or hydrophobic). These polymers are generally known as xe2x80x9cgrafting copolymersxe2x80x9d or xe2x80x9ccopolymers with a comb structurexe2x80x9d.
All these copolymers are known in the art and may be prepared according to processes such as free-radical polymerization followed by a step of polymerization-grafting, anionic or cationic polymerization, polycondensation, or alternatively according to a recent technique known as xe2x80x9ccontrolled free-radical polymerizationxe2x80x9d described, inter alia, in xe2x80x9cNew Method of Polymer Synthesisxe2x80x9d, Blackie Academic and Professional, London, 1995, volume 2, page 1, or in Trends Polym. Sci. 4, page 183 (1996) by C. J. Hawker, and especially by atom-transfer free-radical polymerization described in JACS, 117, page 5614 (1995), by Matyjasezwski et al.
The amphiphilic copolymers comprising at least one hydrophilic block and at least one hydrophobic block, used for the stabilization of dispersions of particles of insoluble UV screening agents in a liquid medium, are preferably nonionic polymers. Specifically, given that the predispersions and the cosmetic compositions prepared therefrom generally contain an aqueous phase, the presence of many charges on the dispersing polymer would excessively increase its solubility in the dispersing medium and would consequently promote the desorption at the time of dilution with a cosmetically acceptable medium.